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Comparative Yield Method

The comparative yield method was developed by Haydock and Shaw in Australia during the 1970's as a rapid method to determine total biomass when sampling in quadrats. Species biomass is not directly evaluated, but a concurrent estimate of species composition is usually obtained by the dry-weight-rank method. Depending on the objectives of the study, the comparative yield method can be used to describe biomass property. It can be applied to a wide variety of vegetation types, particularly grasslands and shrublands, but becomes more complicated in vegetation with a diverse array of species or life-forms.

Unlike the weight-estimate method or the doubling sampling method, the comparative yield method the difficulty of determining absolute biomass values. Instead, biomass is scored relative to a set of reference quadrats that are established at the start of sampling. The comparative yield method consists of three stages.

1. Selection of Reference Quadrats - a set of 5 quadrats are established at the start of sampling to reflect the range of biomass scores that will be assigned during sampling, with Score 1 representing a typical low-biomass quadrat, Score 5 representing a typical quadrat in a high-biomass area, and Scores 2, 3, and 4 as intermediate biomass at the site. These quadrats may be clipped and weighed as a training exercise, until a set is obtained that shows a constant increment between quadrats (i.e. indicating a linear biomass relationship). The precision of the method is compromised when the biomass of reference quadrats departs from a linear relationship. To ensure a representative reference, a new set of quadrats should be established at each new site.
2. Assessment of Sample Quadrats - scores are assigned to each quadrat during field sampling, keeping in mind the biomass levels that were present in the set of reference quadrats. Half-scores and scores greater than 5 should be assigned when necessary. Quadrat scores are estimated on the basis of dry matter content, which is often difficult when faced with quadrats that include different growth forms and stages of phenology. Failure of data from the sample quadrats to follow a normal distribution indicates poor selection of sample unit size and/or reference quadrats.
3. Collection of Calibration Quadrats - at the conclusion of sampling, another set of quadrats is scored, clipped, dried, and weighed. The number of samples selected for the calibration data set depends on the observer's ability to furnish accurate visual estimates and the variability of the biomass estimates, but should encompass the range of biomass values and the majority of species encountered during sampling (n ~ 10-20 quadrats). Regression analysis is used to compare scores and harvested values of the calibration samples, which allows data collected from the sample quadrats to be converted to actual biomass. A regression equation reflecting a close linear relationship (preferably passing through the origin), will ensure an accurate and precise biomass estimate.

The comparative yield method is regularly used for rangeland inventory or monitoring applications where only large differences in biomass need to be identified, such as evaluating range condition or carrying capacity. Furthermore, precision is improved by large sample sizes, so it is more efficient to quickly rank many quadrats rather than linger over the correct rank for any one quadrat. Some observers find it beneficial to carry a set of photographs illustrating the reference quadrats during assessment of the sample quadrats.

Data is usually collected from multiple quadrats located along a transect, so that the sample unit is each transect. Therefore, data must be collected from several transects to determine the precision of the sample, for statistical analysis of biomass data.

References and Further Reading

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Bureau of Land Management. 1996. Sampling vegetation attributes. Interagency Technical Reference, BLM/RS/ST-96/002+1730. pp 112-121.

Despain, D.W., and E.L. Smith. 1989. The comparative yield method for estimating range production. In: G.B. Ruyle. (ed). Some methods for monitoring reangelands and other natural vegetation. Arizona Cooperative Extension Service Report No. 9043. pp 65-83.

Friedel, M.H., Chewings, V.H., and G.N. Bastin. 1988. The use of comparative yield and dry-weight-rank techniques in arid rangelands. Journal of Range Management 41:430-434. (pdf)

Friedel, M.H., and G.N. Bastin. 1988. Photographic standards for estimating comparative yield in arid rangelands. Australian Rangelands Journal 10:34-38.

Haydock, K.P., and N.H. Shaw. 1975. The comparative yield method for estimating the dry matter yield of pasture. Australian Journal of Experimental Agriculture and Animal Husbandry 15:663-670.

Tadmor, N.H., Brieghet, A., Noy-Meir, I., Benjamin, R.W., and E. Eyal. 1975. An evaluation of the calibrated weight-estimate method for measuring production in annual vegetation. Journal of Range Management 28:65-69. (pdf)